It is the general practice in construction machines like hydraulic power shovels to have an upper rotary body, which is rotatably mounted on a lower mobile vehicular body, rotationally driven from a hydraulic drive circuit which constitutes an inertial body drive mechanism for the upper rotary body.
In this regard, FIGS. 11 and 12 show an example of prior art hydraulic rotational drive circuit of the sort mentioned above, namely, an inertial body drive for rotationally driving a hydraulic power shovel.
In these figures, indicated at 1 is a hydraulic motor for rotational drive, which is connectible to a hydraulic pump 2, which serves as a hydraulic pressure source, and to a tank 3, through conduits 4 and 5 from the pump 2 and the tank 3 and through first and second main conduits 6A and 6B. The hydraulic motor 1 is rotationally driven by charging and discharging the oil pressure which is supplied from the hydraulic pump 2, and as a result the inertial body or the upper rotary body is put in rotation on the lower mobile vehicular body (both upper and lower bodies not shown).
Indicated at 7 is a directional switch valve which is interposed to connect the main conduits 6A and 6B selectively with the conduit 4 from the pump 2 or the conduit 5 to the tank 3. The directional switch valve 7 has its ports on the side of the hydraulic pressure source connected to the hydraulic pump 2 and tank 3 through the pump and tank conduits 4 and 5, and its ports on the side of the motor connected to the hydraulic motor 1 through the first and second main conduits 6A and 6B, respectively. The directional switch valve 7 is provided with a manual lever 7A to be operated by the operator to shift the valve from a neutral position A to either one of left and right drive positions B and C, which control the direction of the oil pressure to be supplied to the motor 1 and from the hydraulic motor 1, namely, the directions of oil pressure flows to and from the hydraulic motor 1. When returned to the neutral position A, the directional switch valve 7 blocks communication of the pump and tank conduits 4 and 5 with the main conduits 6A and 6B to stop oil pressure flows to and from the hydraulic motor 1.
Indicated at 8A and 8B are a pair of charging check valves which are connected to the main conduits 6A and 6B at halfway points between the hydraulic motor 1 and the directional switch valve 7. These check valves 8A and 8B are connected to the tank 3 through an auxiliary conduit 9 and a tank conduit 10 to supply the operating oil in the tank 3 to the main conduits 6A and 6B when a negative pressure is developed in these conduits 6A and 6B, for example, at the time of inertial rotation of the hydraulic motor 1.
Denoted at 11A and 11B are a pair of overload relief valves which are connected to the main conduits 6A and 6B at halfway points between the hydraulic motor 1 and the directional control valve 7. These overload relief valves 11A and 11B are connected to the tank 3 through the auxiliary conduit 9, and at the same time to the oil inlets of the check valves 8A and 8B, respectively. The overload relief valves 11A and 11B are calibrated to open at a predetermined control pressure level PC (see FIG. 12) which is determined by a valve spring 12A or 12B. When an excessive pressure, that is, a pressure in excess of the predetermined control pressure level PC develops in the main conduit 6A (or 6B), for example, at the time of inertial rotation of the hydraulic motor 1, the overload relief valve 11A (11B) is opened to relieve the excessive pressure into the opposite main conduit 6B (6A) through the check valve 8B (8A), thereby limiting the maximum pressure in the main conduits 6A and 6B to the predetermined control level PC.
Indicated at 13 is a drain conduit which is connected to the hydraulic motor 1, the drain conduit 13 serving to return to the tank 3 the drain oil resulting from partial leakage of the pressure oil supplied to the hydraulic motor 1.
In this prior art arrangement, when the directional switch valve 7 is switched from the neutral position A to the drive position B, the oil pressure from the hydraulic pump 2 is supplied to the hydraulic motor 1 through the main conduit 6A to rotationally drive the upper rotary body, which is an inertial body, by the hydraulic power of the motor 1, for example, in the clockwise direction. At this time, the return oil from the hydraulic motor 1 is continuously discharged into the tank 3.
In this state, if the directional switch valve 7 is returned to the neutral position A from the drive position B in order to stop the rotation of the upper rotary body, the supply of oil pressure from the hydraulic pump 2 to the hydraulic motor 1 through the main conduit 6A is blocked, relieving the upper rotary body of the driving force by the hydraulic motor 1.
However, the upper rotary body, which is under the influence of the inertial force as an inertial body, puts the hydraulic motor 1 in inertial rotation to continue its pumping action, so that the pressure oil from the main conduit 6A is discharged into the other main conduit 6B. Then, as the pressure on the side of the main conduit 6A turns to a negative level by the inertial rotation, the operating oil in the tank 3 is replenished into the main conduit 6A through the tank conduit 10 and check valve 8A.
As a result, a relatively large amount of pressure oil is sealed in the main conduit 6B between the hydraulic motor 1 and the directional switch valve 7, thereby generating a braking pressure to stop the inertial rotation of the hydraulic motor 1. As indicated by characteristics curve 14 shown in FIG. 12, the overload relief valve 11B is opened against the action of the spring 12B as soon as the braking pressure exceeds the control pressure level PC at which the overload relief valve 11B is designed to be opened, relieving the braking pressure in the main conduit 6B into the main conduit 6A through the auxiliary conduit 9 and check valve 8A. Consequently, the inertial rotation of the hydraulic motor 1 is gradually braked, and the hydraulic motor 1 as well as the upper rotary body comes to a stop at a time point t1 (FIG. 12) when the overload relief valve 11B is closed by the action of the spring 12B.
In case of the above-described prior art, a braking pressure is produced in the main conduit 6B by the inertial rotation of the hydraulic motor 1, causing the overload relief valve 11B to open for braking the inertial rotation of the hydraulic motor 1. After braking the inertial rotation, the overload relief valve 11B is closed at a time point t1 as shown in FIG. 12, so that, once the hydraulic motor 1 is stopped, the pressure in the main conduit 6B is at a relatively high level as indicated by the characteristics curve 14 in FIG. 12. On the other hand, the pressure in the main conduit 6A remains at a low level until the time point t1 as indicated by the characteristics curve 15 in FIG. 12.
Therefore, according to the prior art, even if the hydraulic motor 1 is once stopped at the time point t1 of FIG. 12, it is likely that a relative large pressure differential .DELTA.P exists between the main conduits 6B and 6A as indicated by the characteristics curves 14 and 15. Due to this pressure differential .DELTA.P, the hydraulic motor 1 tends to rotate in a reverse direction, namely, in a direction reverse to the direction of the above-mentioned inertial rotation, and as a result the oil pressure starts to flow toward the main conduit 6A from the main conduit 6B in such a way as to diminish the pressure differential .DELTA.P gradually.
However, at this time, despite the gradual diminishment of the pressure differential .DELTA.P, inertial force of reverse direction is applied to the upper rotary body by the reverse rotation of the hydraulic motor 1, and as a result the hydraulic motor 1 continues its reverse rotation. Therefore, this time the pressure in the main conduit 6A, indicated by the characteristics curve 15 of FIG. 12, becomes higher than the pressure in the main conduit 6B of the characteristics curve 14 at the time point t2, putting the hydraulic motor 1 in rotation again in a reversed direction.
In this manner, according to the prior art, at the time point t1 which coincides with the end of the open phase of the overload relief valve 11B (11A), the hydraulic motor 1 which has been in an inertial rotation is temporarily stopped by the increase of the pressure differential .DELTA.P between the main conduits 6A and 6B. However, then the hydraulic motor 1 begins to rotate again in a reversed direction, thereafter repeating the reversed rotations together with the upper rotary body.
In order to solve this problem, a number of proposals have been made, for example, as in Japanese Laid-Open Patent Specifications 57-25570 and 58-91902, employing a swingback preventive valve in each of the main conduits which are connected to the hydraulic motor, thereby to prevent the swingback motions of the inertial body. In this case, however, the swingback preventive valve has to be provided separately in each of the first and second main conduits, giving rise to a number of problems such as a marked increase in the number of component parts required and complication of the piping work, in addition to the problem that the hydraulic circuit as a whole becomes objectionably large in size. Further, the prior art swingback preventive valves for the inertial body are each in the form of a poppet valve, which maintains a closed state by holding a valve body substantially in linear contact with a valve seat, so that it can be inadvertently opened even by slight fluctuations in the drive pressure of the hydraulic motor in the braking pressure, hindering smooth drive or stop motions of the inertial body.
Further, in this connection, Japanese Laid-Open Patent Specification 57-1803 discloses an apparatus for preventing reversing motions of the inertial body, including a spool type on-off valve located between first and second main conduits which are connected to a hydraulic motor, a pair of oil chambers formed at the opposite ends of the spool of the on-off valve, and a pair of springs provided in the respective oil chambers to bias the spool toward a neutral position, a pressure (pilot pressure) from the main conduits being introduced into the respective oil chambers through a throttle.
In this case, however, either the hydraulic motor drive pressure which is produced in one of the main conduits or the braking pressure which is produced in the other main conduit continuedly acts in the oil chamber of the on-off valve as a pilot pressure through the above-mentioned throttle. Therefore, regardless of the reversing motions of the hydraulic motor caused by the inertial body, the spool of the on-off valve can be opened against the action of the spring when the pilot pressure from the main conduit becomes higher. It follows that the on-off valve disclosed in the above-mentioned Japanese Laid-Open Patent Specification No. 57-1803 has possibilities of hindering smooth operations in driving and stopping the inertial body, failing to provide improvements in reliability and safety of the reverse motion preventive apparatus for the inertial body.
In view of the above-discussed problems or drawbacks of the prior art, it is an object of the present invention to provide a hydraulic inertial body drive mechanism including a valve for preventing reversing motions of an inertial body, the inertial body drive mechanism being arranged to prevent effectively the repeated reversing motions of a hydraulic motor which would normally take place after stopping inertial rotation of the hydraulic motor, thereby enhancing the degree of safety and reliability of operation while reducing the number of component parts to simplify the construction of the drive mechanism as a whole.